This invention relates to digital wireless communications systems and, more particularly, to transmission channels subject to disturbances such as frequency-selective fading and multipath effects.
In high data throughput wireless digital communication systems, the maximum data transmission rate may be limited by disturbances in the wireless propagation path (i.e., the communication channel). These effects include disturbances such as frequency-selective fading and multipath (copies of the transmitted signal delayed in time to the receiver). These disturbances may result in interference between the digitally-modulated symbols representing the information bits to be transmitted, thus impairing the receiver's demodulator from correctly decoding the received symbols to arrive at accurate bit decisions. This “intersymbol interference” may cause the received symbols to overlap the decision boundaries in the complex signal space to adjacent symbols and result in either bit decision errors or lowered bit decision confidence. Systems subject to these effects may thus be required to operate at lower data throughput rates or higher error rates than would otherwise be attainable.
A traditional solution to such channel disturbance problems is to provide an adaptive equalizer consisting of a digital filter whose coefficients can be adjusted to model the inverse of the actual channel impulse response. The resulting digital filter thus enables compensation for the effects of channel nonlinearity by providing this reciprocal of the actual channel impulse response (e.g., a polarity-inverted representation of the channel transmission characteristics as degraded by whatever such disturbances are actually present at a particular time). The determination of the channel impulse response is typically performed by transmitting a test pattern (i.e., a training sequence) to excite the channel at all frequencies, or all frequencies of significant interest, within the data bandwidth of interest and measuring the resulting effect on the training sequence waveform upon transmission through the channel. The calculation of filter coefficients to model the channel impulse response based upon this measurement has typically been done using an estimation process such as a Mean Square Error algorithm.
In digital transmission systems operating at high data rates (such as military communications systems or for commercial wireless Internet access), the channel impulse response estimation time (i.e., the time required to provide such filter coefficients) becomes a critical factor. Since the disturbance effects may be constantly changing, the impulse response estimate must be updated frequently to accommodate high data rates. As a result, the time required to calculate the channel impulse response may become an important factor limiting the maximum data rate of the system. Established techniques and methods for calculating the channel impulse response have typically been subject to constraints on speed, accuracy, security or other relevant factors.
In applications in which security of transmitted information is important, it is desirable that a training signal have characteristics such that, when transmitted, it is substantially undetectable to provide a high level of security and, when received, it is readily usable by processing which can be employed rapidly and with limited complexity. Thus, in addition to security, avoidance of transmission speed constraints resulting from processing requiring multiplication or division of complex mathematical functions is desirable.
Objects of the present invention are, therefore, to provide forms of training sequences and communication systems and methods which are new or improved and which may provide one or more of the following capabilities or characteristics:                provision of improved forms of training sequence;        multiple channel impulse response estimations from processing of a training sequence transmitted with a data packet;        increased accuracy of results by averaging values of multiple channel impulse response observations;        provision of channel impulse response estimations without requirement for multiplication or division of complex functions;        improved capability to estimate channel impulse response;        rapid estimation of channel impulse response;        system initialization with enhanced security via changing training sequence coding.        